Research On Doping And Sodium Compensation Of Tunnel Structured Sodium Manganate

Lithium-ion batteries（LIBs）are widely used in consumer electronics and electric vehicles,and related research has also attracted widespread attention.However,due to the increasing depletion of lithium resources,its price becomes more expensive,so it is not suitable for large-scale energy storage.Sodium-ion batteries are emerging as an alternative for large-scale energy storage and portable applications due to their abundant resources,lower cost,and similar chemical properties to lithium.The tunnel-structured Na_0.44MnO₂（NMO）,as one of the cathode materials of sodium ion batteries,has the characteristics of fast carrier migration and strong structural stability.This benefits from the large S-shaped tunnel composed of MnO₆ octahedrons and MnO₅ square pyramids,and therefore stands out among many cathode materials.However,the poor electrical conductivity,as well as Jahn-Teller effect and the dissolution of Mn²⁺,make the rate and cycle performance still need to be further improved.In addition,the lower the initial coulombic efficiency of NMO,the less available capacity it has in practical application of a full battery.The main research results of this thesis on the above issues are as follows:1.Different Mn sources are used to prepare sodium manganate materials through physical and chemical characterization.It is found that the phase was the purest when Na/Mn（molar ratio）is around 0.5 for manganese oxides as the precursors,and the tunnel structure could still be maintained when sintered at 900℃;The phase is the purest when Na/Mn（molar ratio）is around 0.44 for manganese carbonates as the precursors,and the suitable sintering temperature is 800℃.Considering that Nb is easier to be doped into the bulk phase at 900℃,and after comparing the price and comprehensive performance,Mn₂O₃ and Na HCO₃ are finally selected as the precursors of sodium manganate cathode materials,and used as the optimal comparison group for subsequent research on the modification of sodium manganate.2.Nb-doping sodium manganate was prepared by high-temperature solid-phase sintering at 900℃.This is the first time to study the effect of high valence Nb⁵⁺on the electrochemical performance of sodium manganate after replacing Mn sites.Electrochemical tests show that NMO@2%Nb has ultra-high rate performance and good cycle performance,the specific capacity is still about 81.51 mAhg^-1 at 30 C,and the capacity retention rate is 82.3%at 5 C after 800 cycles.According to the change of voltage drop and electrochemical impedance spectroscopy（EIS）,the rapid migration of ions after Nb-doping is explained in the kinetics.The analysis of the energy storage mechanism based on the first principles calculations shows that:（1）Nb-doping makes the tunnel pore size expand,which is conducive to the migration of Na⁺;（2）The density of states calculation shows that the band gap of NMO@2%Nb is smaller than sodium manganate,which means that the electronic conductivity is improved.3.On the basis of Nb-doping,a lot of trial and error is needed to find a suitable cathode self-sacrificial sodium compensation additive to improve the initial week coulombic efficiency of sodium manganate.Finally,10wt% NaCrO₂ sodium compensation additive was selected.While greatly improving the initial week coulombic efficiency,the cycle performance of sodium manganate is further improved,and the capacity retention rate is increased to 87.4%at5 C after 800 cycles.Through various physical and chemical characterizations,we analyzed the reasons for the improved cycle performance and the release ability of Na⁺ in NaCrO₂ to prove that NaCrO₂ has a good sodium compensation effect.